SYDNEY, Dec 6 (Reuters) - Three of Australia's "Big Four" banks have raised their lending interest rates in less than a week after citing squeezed margins, risking a political backlash by hiking outside the central bank's rate-setting cycle.

The increases were announced ahead of Tuesday's Reserve Bank of Australia meeting, where the cash rate was held at a record low of 1.5 percent, and add to a perception that big banks use their market power to squeeze customers.

"The amount of (margin) they're making on lending is the tightest it's been ever," Steven Munchenberg, chief executive of the Australian Bankers Association, told Reuters by phone.

"In that context I think the banks are trying to adjust some of the pricing to reflect the realities of the costs in the market."

National Australia Bank Ltd, Westpac Banking Corp and Commonwealth Bank of Australia each lifted lending rates for some mortgage products between 6 and 15 basis points, according to separate statements.

The four banks together control roughly 80 percent of the Australia's lending market and came under intense political pressure in August - culminating in a parliamentary inquiry - when they failed to pass on a central bank cut in full.